This disclosure pertains to plant-sourced carbon. In particular, this disclosure relates to poly-(caffeyl alcohol) (“PCFA”), also named as C-lignin as a source for carbon.
Carbon fibers are a high volume high performance product in applications ranging from carbon fiber reinforced epoxy for aerospace and marine applications, electromagnetic interference shielding, biomedical applications for regenerative medicine and cancer treatment, energy storage devices and water filtration. Recently, concerns about greenhouse gas emissions and climate change have motivated a shift to lighter automobiles. To this end, significant efforts are being focused on the development and deployment of carbon fiber-reinforced composites. Modeling studies have indicated that over 60% of the steel in a vehicle could be replaced by carbon fiber-reinforced composite materials, dramatically reducing its weight while maintaining the vehicle's impact protection. Furthermore, for every 10% reduction in weight of the vehicle, the fuel economy is estimated to increase by 6%.
Carbon fiber composites (CFCs) display several properties that are very attractive in structural applications: high strength and stiffness, low density, they are chemically inert and show high electrical and thermal conductivity. However, methods for producing these CFCs are less than ideal. Currently, carbon fiber is manufactured predominantly from polyacrylonitrile (PAN) with a small fraction originating in pitch. PAN based on the acrylonitrile monomer has a high cost. Pitch raw materials are cheaper but the processing involves cleanup leading to high final cost. Pitch from petroleum is preferred over coal pitch from raw material clean up perspectives, but needs vacuum cleaning to remove volatile matter. To form carbon fibers, wetting of PAN prior to carbonization is employed. Typical carbon yields for PAN-based and pitch-based carbon fibers are about 50-60% and 70-80% respectively. A pre-oxidation step to carbonization has been shown to result in higher carbon yield, and additional graphitization with argon has increased the carbon yield to 80% for PAN fibers.
Synthetic polymers such as polyacetylene, polyethylene, and polybenzoxazole have also been investigated as a potential route for obtaining carbon fibers. While the strength to weight ratio of these polymers exceeds that of glass, the cost/weight ratio remains prohibitive. Thus, fiberglass based composites remain the high volume product. This raises further environmental concerns as the carbon footprint for producing fiberglass is prohibitive. Because of such concerns, development of a source of carbon fiber based on plant material is being strongly promoted.
Kraft lignin, extracted from hardwoods, has been extensively studied as a feedstock for biomaterials. To facilitate the melting of the lignin, organic solvent based extraction, chemical treatment or melt blending are employed. The value of lignin as a source for carbon fibers obtained from melt and dry spinning of hardwood Kraft lignin (HKL), softwood Kraft lignin (SKL) and alkali softwood Kraft lignin has been shown. Hydrogenation with NaOH using Raney-Ni, followed by steam explosion to isolate the lignin and then modification to lower its softening point, thereby facilitating melt spinning of the fibers, has been used. However, this method was expensive and a cheaper alternative was attempted using creosote for phenolysis. Although phenolysis improved the yield to 40%, tensile properties were low when compared to hydrogenation. Acetic acid pulping from hardwood gave fusible lignin that could be melt-spun. Lignin from softwood resulted in a high fraction of high molecular weight infusible lignin, that must be separated from the fusible fraction in order to facilitate melt spinning.
Chemo-enzymatic treatment (sulfonication) has been shown to transform water insoluble Kraft and organosolv lignins to water soluble material, and facilitates grafting of acrylic compounds onto the lignin backbone. Esterification of lignins from sources such as palm trunk, poplar, maize, barley, wheat, and rye with succinate anhydride showed relatively lower substitution of succinate, but gave thermal stability ranging from 100 to 600° C., with the highest for lignin from rye.
Blending polymers with lignin enables fiber integrity through improved melt strength. Poly(ethylene oxide) (PEO) has been widely studied for imparting ability for spinning lignin into fibers. Incorporation of 5% and 3% PEO in hardwood Kraft lignin (HKL) improved spinning capability and tensile properties, respectively. With an Alcell/PEO blend, strong hydrogen bonding results in miscible blends aiding spinning of fibers, although addition of PEO did not improve the mechanical properties of the fiber. To overcome brittleness, lignin was blended with polyethylene terephthalate (PET) and polypropylene (PP). Blends of PET and PP with HKL gave fiber diameter ranges from 30 to 76 μm, and blends with 25% polymers yielded 60% carbon after carbonation; however, this route did not improve the physical properties of the fibers. Similarly, polyethylene glycol (PEG)-lignin was used for single needle melt spinning to obtain 23 μm diameter fibers at 170° C. and PVA by researchers in the field.
The above examples clearly demonstrate that considerable processing is necessary to obtain high carbon yields, good spinnability and useful fiber properties from typical bulk lignin, such as the Kraft lignin obtained as a by-product from the pulp and paper industry.